EP3077082A1

EP3077082A1 - Catalytic oxidation of a gas comprising hydrogen sulfide

Info

Publication number: EP3077082A1
Application number: EP14803167.7A
Authority: EP
Inventors: Morten Thellefsen
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 2013-12-02
Filing date: 2014-11-28
Publication date: 2016-10-12
Anticipated expiration: 2034-11-28
Also published as: EA201691024A1; WO2015082351A1; MY185568A; EP3077105A1; EP3077105C0; US9776133B2; PL3077105T3; WO2015082352A1; ES3019784T3; CN105792927A; CA2932289C; EP3077082B1; EP3077105B1; EA032562B1; CA2932289A1; CN105992633A; CN105792927B; CN105992633B; US20160296889A1

Abstract

The present invention in a broad form relates to a process for oxidation of a gas comprising at least 0.05 vol% hydrogen sulfide and at least 0.05 vol% of the sum of carbon monoxide, carbonyl sulfide and carbon disulfide, comprising the steps of (a) directing said gas to contact, at a first temperature, a first material catalytically active in oxidation of hydrogen sulfide at said first temperature, providing a heated and partially oxidized gas, and (b) directing said partially oxidized gas to contact, at a second higher temperature, a second material catalytically active in oxidation of carbon monoxide, carbon disulfide and/or carbonyl sulfide providing an incinerated gas characterized in said second material being catalytically inactive in the oxidation of carbon monoxide and/or carbonyl sulfide at said first temperature and said second material being catalytically active in the oxidation of carbon monoxide and/or carbonyl sulfide at said second temperature with the associated benefit of reducing the requirements for inlet temperature as well as reducing the catalyst cost by reducing the required volume of expensive material catalytically active in oxidation of carbon monoxide and carbonyl sulfide, typically comprising noble metal.

Description

CATALYTIC OXIDATION OF A GAS COMPRISING HYDROGEN SULFIDE

The invention relates to catalytic oxidation of gases com- prising ¾S and at least one of CO, COS and C S 2 , in which the concentration of ¾S is at least 0.05 vol% and the sum of concentrations of CO, COS and C S 2 gases is at least 0.05 vol%. Such lean gases could e.g. be a waste stream from the production of viscose fibers or other viscose products. H₂S, CO, COS and C S 2 gases are toxic and must be treated before emitted to the atmosphere. A commonly used method is to oxidize these species to S O2 , CO2 and H₂0 and subsequent^¬ ly remove the S O2 from the gas. The S O2 can be removed by conversion into sulfuric acid by catalytic conversion of S O2 to S O3 , reaction with water va^¬ por and condensation of the sulfuric acid vapor, but also S O2 scrubbing methods can be used, e.g. reaction with lime to form gypsum or reaction with sodium hydroxide to form sodium sulfite/sulfate.

The conversion of ¾S and C S 2 in the lean gas into S O2 can be by thermal oxidation at temperatures around 850 - 1000 °C. Due to the low calorific value of the lean gas, thermal oxidation requires heating the lean gas, typically by com^¬ bustion of significant amounts of support fuel, which in^¬ creases the cost of the process.

Catalytic oxidation of ¾S and C S 2 can proceed at much low- er temperature and is thus much more economic for lean ¾S and COS+C S 2 gases. According to the prior art (e.g. AT398785 B) , H₂S, CO, COS and CS₂ are catalytically oxidized on a noble metal promot^¬ ed catalyst; the expensive noble metal, platinum, is usual^¬ ly the active component of the catalyst, e.g. in combina- tion with copper and manganese oxide on a carrier such as silica .

However, according to the present disclosure, a catalyst system is disclosed, in which a sequential process results in a lower required gas temperature and a reduced amount of noble metal promoted catalyst. This catalyst system employs the fact that the exothermal oxidation of ¾S is initiated a temperatures as low as 200°C on a metal oxide oxidation catalyst, and that the oxidation of ¾S (and partial oxida- tion of CS₂ to COS/CO) increases the temperature to at least 320°C, which is required for oxidation of CS₂,COS and CO on a catalyst promoted with palladium, Pd.

In the present application the term "a material being cata- lytically active" e.g. in the oxidation of hydrogen sulfide at a given temperature, shall be understood as a material, in the presence of which the reaction rate of the mentioned reaction (here oxidation of hydrogen sulfide) is increased by at least an order of magnitude, preferably 2 orders of magnitude, and even more preferably 5 orders of magnitude when compared to the gas phase reaction.

Similarly in the present application the term "a material being catalytically inactive" e.g. in the oxidation of car- bon monoxide at a given temperature shall be understood as a material, in the presence of which the reaction rate of the mentioned reaction (here the oxidation of carbon monox- ide) is not similarly increased compared to the gas phase reaction. As will be understood by the skilled person, the term catalytically inactive with respect to a reaction shall be understood as having a very small activity with respect to that reaction, compared to other reactions, but not as absolutely inert, since all materials will show some catalytical activity.

Where concentrations are stated in % (or v/v %) this shall be understood as volumetric %. Concentration of a specific constituent may for brevity be symbolized by c_Co i.e. with the chemical formula of that constituent as subscript.

A lean gas (such as a lean ¾S gas or a lean viscose off gas) shall be understood as a gas having a concentration of ¾S and CS₂ below the lower explosion limit, which corre^¬ sponds to less than 1.25% CS₂ and less than 4.3% ¾S.

A metal oxide oxidation catalyst shall be understood as a catalyst comprising a metal oxide catalytically active in oxidation .

The present invention in a broad form relates to a process for oxidation of a gas comprising at least 0.05 vol% hydro- gen sulfide and at least 0.05 vol% of the sum of carbon monoxide, carbonyl sulfide and carbon disulfide, optionally in combination with a hot gas comprising the steps of

(a) directing said gas to contact, at a first temperature, a first material catalytically active in oxidation of hy- drogen sulfide at said first temperature, providing a heat^¬ ed and partially oxidized gas, and (b) directing said partially oxidized gas to contact, at a second higher temperature, a second material catalytically active in oxidation of carbon monoxide, carbon disulfide and/or carbonyl sulfide providing an incinerated gas characterized in said second material being catalytically inactive in the oxidation of carbon monoxide and/or carbonyl sulfide at said first temperature and said second mate^¬ rial being catalytically active in the oxidation of carbon monoxide and/or carbonyl sulfide at said second temperature with the associated benefit of reducing the requirements for inlet temperature (or the amount of hot gas added e.g. by a combusted support fuel) as well as reducing the cata^¬ lyst cost by reducing the required volume of expensive ma^¬ terial catalytically active in oxidation of carbon monoxide and carbonyl sulfide, typically comprising noble metal.

In a further embodiment said first temperature is between 200 and 320°C, and preferably between 220 and 300°C, with the associated effect of this temperature being suitable for ignition of materials such as ¾S over an appropriate first catalytically active material.

In a further embodiment said second temperature is between 320 and 380°C preferably between 330 and 360°C with the as- sociated effect of this temperature being suitable for ig^¬ nition of materials such as CO and COS over an appropriate second catalytically active material.

In a further embodiment the amount of hydrogen sulfide is sufficient for raising the temperature to said second tem^¬ perature by the heat developed from the oxidation of hydro^¬ gen sulfide without combining the gas with said hot gas with the associated benefit of reducing the amount of ener^¬ gy provided by support firing.

In a further embodiment said first catalytically active ma- terial comprises one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper, on a support comprising silicon carbide or one or more oxides of metals taken from the group of aluminum, silicon and tita- nium with the associated benefit of said materials being effective in the catalytic oxidation of hydrogen sulfide at low temperatures.

In a further embodiment said second catalytically active material comprises a platinum group metal, preferably pal^¬ ladium, in combination with one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper, on a support comprising silicon carbide or one or more oxides of metals taken from the group of aluminum, silicon and titanium with the associated benefit of said materials being active in the catalytic oxidation of carbon monoxide and/or carbonyl sulfide. In a further embodiment one or both of said first and sec^¬ ond catalytically active material is provided as a monolith structure with the associated benefit of providing an ac^¬ tive catalyst with low pressure drop. In a further embodiment the total concentration of ¾S is less than 2.5 vol% with the associated benefit of ensuring that the oxidation catalyst temperature remains below the deactivation temperature of about 550°C, at which catalyti^¬ cally active material may become physically unstable.

In a further embodiment the concentration condition c_Cs2 + 0.5 Ccos + 0.25 c_Co < 0.7 vol% is satisfied with the associ^¬ ated benefit of ensuring that the catalytically active ma^¬ terial temperature remains below the deactivation tempera^¬ ture of about 550°C. A further aspect of the invention relates to a process for production of sulfuric acid comprising the further steps of (c) directing said incinerated gas to contact a material catalytically active in oxidation of sulfur dioxide to sul^¬ fur trioxide providing an oxidized gas and

(d) at least partially hydrating sulfur trioxide forming sulfuric acid,

(e) withdrawing from said oxidized gas sulfuric acid, with the associated benefit of providing a process for pro^¬ duction of sulfuric acid from lean waste gases, which is cost and energy efficient.

In a further embodiment the process comprises the step of condensing sulfuric acid in a cooled condenser with the as^¬ sociated benefit of providing a process for production of sulfuric acid, which is efficient.

In a further embodiment the process comprises the step of absorbing sulfur trioxide in sulfuric acid with the associ^¬ ated benefit of providing a process for production of sul- furic acid, which is efficient. A further aspect of the invention relates to a process plant configured for directing a gas to a catalyst system, characterized in said catalyst system comprising a material catalytically active in oxidation of hydrogen sulfide but substantially catalytically inactive in oxidation of car- bonyl sulfide and carbon monoxide and a further material catalytically active in oxidation of carbonyl sulfide and carbon monoxide at a second higher temperature with the as^¬ sociated benefit of said process plant providing an econom- ically beneficial catalyst configuration.

In a further embodiment one or both of said catalytically active materials comprises one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper on a support comprising silicon carbide or one or more ox^¬ ides of metals taken from the group of aluminum, silicon and titanium with the associated benefit of said materials being effective in the catalytic oxidation of hydrogen sul- fide at low temperatures.

In a further embodiment said second catalytically active material comprises a platinum group metal, preferably pal^¬ ladium in combination with one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper on a support comprising silicon carbide or one or more ox^¬ ides of metals taken from the group of aluminum, silicon and titanium with the associated benefit of said materials being active in the catalytic oxidation of carbon monoxide and/or carbonyl sulfide. In general it is preferred to use catalytically active ma^¬ terials, which do not comprise copper or manganese since these two metal have a tendency to form sulfates (cf . J. Phys .Chem, 95, 4051 (1991)).

The following reactions proceed on the catalyst surface and to some extent in the gas phase, i.e. without catalyst.

1) H₂S + 1.5 0₂ -> S0₂ + H₂0 + 124 kcal/mol

2) CS₂ + 1.5 0₂ -> COS + S0₂ + 132 kcal/mol

3) COS + 1.0 0₂ -> S0₂ + CO + 64 kcal/mol

4) CO + 0.5 0₂ -> C0₂ + 68 kcal/mol

In the gas phase CS₂ alone will start reacting via reaction 2, when the temperature is around 200°C and H₂S will start reacting via reaction 1 at around 350°C. However, the gas phase oxidation of CS₂ (via reaction 2) is inhibited by the presence of H₂S, which means that CS₂ is oxidized at a much higher temperature than the stated 200 °C if H₂S is present. Reaction 3 and 4 will not react in the gas phase at temper^¬ atures below 400°C.

The heat released from each of the reactions 1-4 increase the temperature of catalyst and gas. If the calorific value of the gas is too high (i.e. too high concentration of es^¬ pecially H₂S or CS₂) , the temperature will increase above the maximum allowable catalyst temperature and thus damage the catalyst. In that case, thermal oxidation must be ap^¬ plied .

For each of the above reactions there is a minimum catalyst temperature, below which the reaction will not proceed. These so-called ignition temperatures for the cata^¬ lyst/reaction combinations are found in Table 1 for the Pt promoted catalyst used according to the prior art and for the catalysts used in the new catalyst system.

Table 1. Minimum ignition temperatures for oxidation catalysts used for oxidation of H₂S, CS₂, COS and CO. The metal oxide oxidation catalyst is not able to oxidise COS and CO. Below 320 °C the noble metal promoted oxidation catalyst is not active for CS₂,COS and CO oxidation.

The heat released by reaction 1 and to some extent reaction 2 can be used to increase the catalyst temperature, such that the minimum temperature for e.g. COS oxidation (360°C) can be met for the Pt based catalyst, even if the tempera^¬ ture at the inlet to the catalyst is below that tempera^¬ ture. If the ¾S concentration is sufficiently high, the heat released by reaction 1 will increase the gas and cata- lyst temperature above the minimum ignition temperature for COS oxidation and reaction 2 and 3 could increase the tem^¬ perature to above 420°C, which again ensures complete oxi^¬ dation of CO.

The present disclosure relates to a catalyst system, which utilizes the heat of reaction to ensure complete oxidation of ¾S and CS₂ at the lowest catalyst cost and at the low^¬ est operating temperature, thus reducing the cost for pre- heating the lean ¾S and CS₂ gas.

Figures

Fig.l illustrates the temperature profile of catalyst and process gas assuming that reactions 1-4 proceed sequential^¬ ly, together with relevant ignition temperatures.

Fig.2 illustrates a sequential catalyst system in a process for oxidation of lean waste gas.

Fig.3 illustrates a process for oxidation of lean waste gas and subsequent hydration and condensation into sulfuric ac^¬ id, employing the disclosed catalyst system. Fig.4 illustrates a process according to the prior art, for oxidation of lean waste gas and subsequent hydration and condensation into sulfuric acid employing a single oxida^¬ tion catalyst. The disclosed catalyst system is shown in Fig.2. The lean ¾S and CS₂ process gas (1) is preheated to minimum 200°C and fed to a first metal oxide oxidation catalyst (2) such that oxidation of ¾S according to reaction 1 and to some extent C S 2 according to reaction 2 takes place. The process gas (3) leaving the metal oxide oxidation catalyst (2) must be above 320 °C before entering the noble metal promoted catalyst (4), for reactions 2, 3 and 4 to proceed to com^¬ pletion. The process gas (5) leaving the noble metal pro^¬ moted catalyst (4) contains S O2 , CO2 and H₂0 products and must be treated further before emitted to the atmosphere, e.g. in a sulfuric acid plant or a scrubber.

The main advantage of this catalyst system is that a rela^¬ tively inexpensive metal oxide oxidation catalyst, being very active in ¾S oxidation, can be used to increase the temperature of the catalyst and process gas to minimum 320 °C. At that temperature the more expensive noble metal promoted catalyst, very active in C S 2 , COS, CO and ¾S oxi^¬ dation can carry out the remaining oxidation. This means that the expensive noble metal promoted catalyst is only used where it is necessary and not for the ¾S oxidation where a less expensive metal oxide oxidation catalyst can catalyze the oxidation. Contrary to this, when using only a Pt-promoted catalyst according to the process of the prior art, expensive Pt is used for oxidation of ¾S, where a less expensive catalyst formulation is used according to the present disclosure.

Furthermore, the metal oxide oxidation catalyst is active at a much lower temperature than the Pt promoted catalyst, i.e. the energy required to heat up a cold lean ¾S and C S 2 gas is reduced and thus lower operating cost is achieved. In a preferred process layout, the disclosed catalyst sys^¬ tem is used for treatment of a lean ¾S and C S 2 gas from viscose production, producing sulfuric acid. The process layout is shown in Fig.3.

Cold lean ¾S and C S 2 gas (6) is received from a viscose producing plant. A fraction of the lean ¾S and C S 2 gas (7) is directed to combustor (11), in which elemental sulfur (9) and natural gas (10) is combusted at minimum 850°C. The hot process gas (12) leaving the combustor (11) is mixed with the cold lean ¾S and C S 2 gas (8), which has bypassed the combustor. The mixed process gas stream (1) is directed to the metal oxide oxidation catalyst (2), on which ¾S and to some extent C S 2 (via reaction (2)) is oxidized. The partly oxidized process gas (3) is then directed to the no^¬ ble metal promoted catalyst (4), over which the unreacted C S 2 , COS and CO is oxidized to CO2 and S O2 . The oxidized process gas (5) leaving the noble metal promoted catalyst (4) is directed to a S O2 reactor (18), in which an S O2 oxi- dation catalyst (19+21) is installed to convert S O2 to S O₃ .

To control the temperature of the second catalyst (21), a heat exchanger (20) is installed. The converted process gas is further cooled in heat exchanger (22) before leaving the S O2 reactor (18) via line (23) . In a special sulfuric acid condenser (26), the process gas is cooled and concentrated sulfuric acid is condensed and withdrawn from the bottom (25) and the cooled clean process gas leaves in the top via line (24) and is emitted via the stack. The cooling medium is atmospheric air, which enters at the top (27) and leaves at the bottom (28) of the condenser (26) . For comparison a process according to the prior art is illustrated in Fig.4. Here the two beds (14, 16) of catalyti^¬ cally active material for oxidizing H2S, CS2, COS and CO are replaced by a single bed of catalytically active mate- rial (30), such as a Pt promoted catalyst, which requires a higher inlet temperature, as well as cooling of the outlet gas .

Example 1

The temperature increase of process gas and catalyst (s) are shown in Fig.l for a process gas with an initial concentra^¬ tion of 0.60 vol% H₂S and 0.30 vol% CS₂. The dashed line (o) presents the disclosed catalyst system with a metal oxide oxidation catalyst to oxidize ¾S and a palladium promoted catalyst for the oxidation of CS2, COS and CO. The solid line (Δ) presents the Pt promoted cata^¬ lyst according to the prior art for carrying out all the chemical reactions.

The horizontal lines correspond to ignition temperatures as shown in Table 1 with e.g. T_H2_S/_M._OX meaning the ignition temperature for ¾S over metal oxide.

With the disclosed catalyst system, reaction 1 (¾S oxida^¬ tion) is carried out over the metal oxide oxidation cata^¬ lyst with 200°C ignition temperature. The inlet temperature is adjusted to 233°C such that the heat evolved during re- action 1 increases the temperature to 320°C, where the no^¬ ble metal promoted catalyst for CS2, COS and CO oxidation becomes active and via reaction 2,3 and 4 increases the temperature to 413°C out of the catalytic reactor.

Reaction 2 can to some extent proceed on the metal oxide oxidation catalyst and in the gas phase, but for process design purposes it is more conservative not to include a contribution to temperature increase on the metal oxide ox^¬ idation catalyst, based on the partial oxidation of C S 2 ac^¬ cording to reaction 2.

With the Pt promoted catalyst according to the prior art it is necessary to increase the inlet temperature to 280°C for the ¾S oxidation to take place. After the ¾S oxidation, the temperature is sufficiently high for the C S 2 , COS and CO oxidation to proceed. The resulting outlet temperature is 460°C well above the optimal temperature for the inlet to the S O2 reactor in a downstream sulfuric acid plant.

Example 2

In the production of viscose fibers, a so-called lean off gas is withdrawn from the production line. The lean off gas is atmospheric air with less than 2.5 vol% ¾S and C S 2 , and typically less than 0.5 vol% of each. Since sulfuric acid is needed for the viscose/ C S 2 production, this lean off gas is treated in a sulfuric acid plant. In the example de^¬ scribed below, extra sulfuric acid production is desired and thus a stream of elemental sulfur is fed to the combus- tor of the sulfuric acid plant. To maintain a sufficiently high temperature in the combustor, a support fuel such as natural gas may also be added, and in other instances all support fuel may be natural gas or another fuel. To save capital cost, a fraction of the lean off gas by^¬ passes the combustor and added to the hot process gas leav^¬ ing the combustor. This both reduces the size of the com^¬ bustor and eliminates the need for cooling the process gas from the combustor to S O2 oxidation temperatures in a waste heat boiler. Furthermore, much less support fuel is re^¬ quired by this process layout.

After mixing the hot process gas from the combustor with the cold bypassed lean gas, the combined process gas con^¬ tains ¾S and C S 2 , which must be oxidized to S O2 , CO2 and H2O before reaching the catalyst for conversion of S O2 to SO₃. Catalytic oxidation is used for this purpose. In Table 2, the difference between a Pt promoted manga^¬ nese/copper oxide oxidation catalyst and the catalyst sys^¬ tem combining a vanadium oxide based catalyst and a palla^¬ dium promoted vanadium oxide based oxidation catalyst is shown. The ignition temperatures of the catalysts are as shown in Table 1, i.e. the catalyst operating temperatures must be above these temperatures to operate efficiently.

Table 2. Operation of H₂S, CS₂,COS and CO oxidation catalysts for a process plant, treating a lean viscose off gas and elemental sulfur for further treatment in a wet type sulfuric acid plant.

In Table 2 it is seen that 14% less natural gas is required for the metal oxide and noble metal promoted catalyst corn- bination, thereby reducing the operating cost. At the same time 10% less lean viscose off gas is directed to the com^¬ bustor, reducing the size of the combustor and thus capital cost. In both cases complete oxidation is observed. Processwise, a further advantage of the metal oxide/noble metal promoted catalyst system is that the temperature af^¬ ter complete oxidation can be kept at a temperature very close to the lower range of the optimum temperature for the SO₂ to SO₃ conversion catalyst. This allows for higher SO₂ conversion efficiency on this catalyst.

The metal oxide/noble metal promoted catalyst system will be a less expensive solution, since the initial oxidation is carried out on a catalyst without expensive noble metals and only the final oxidation is carried out with a noble metal promoted catalyst.

Claims

Claims :

1. A process for oxidation of a gas comprising (8,1) at least 0.05 vol% hydrogen sulfide and at least 0.05 vol% of the sum of carbon monoxide, carbonyl sulfide and carbon di- sulfide, optionally in combination with a hot gas (12) com^¬ prising the steps of

(a) directing said gas (1) to contact, at a first tempera^¬ ture, a first material (2) catalytically active in oxidation of hydrogen sulfide at said first temperature providing a heated and partially oxidized gas (3) , and

(b) directing said partially oxidized gas (3) to contact, at a second higher temperature, a second material (4) cata^¬ lytically active in oxidation of carbon monoxide, carbon disulfide and/or carbonyl sulfide providing an incinerated gas (5)

characterized in said second material (4) being catalyti^¬ cally inactive in the oxidation of carbon monoxide, and/or carbonyl sulfide at said first temperature and said second material being catalytically active in the oxidation of carbon monoxide and/or carbonyl sulfide at said second tem^¬ perature .

2. A process according to claim 1, in which said first temperature is between 200 and 320°C and preferably between 220 and 300°C.

3. A process according to claim 1 or 2, in which said second temperature is between 320 and 380°C preferably between 330 and 360°C.

4. A process according to claim 1, 2 or 3, in which the amount of hydrogen sulfide is sufficient for raising the temperature to said second temperature by the heat devel^¬ oped from the oxidation of hydrogen sulfide without combining the gas (8) with said hot gas (12) .

5. A process according to claim 1, 2, 3 or 4, in which said first catalytically active material (2) comprises one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, and niobium, on a support comprising silicon carbide or one or more oxides of metals taken from the group of aluminum, silicon and titanium.

6. A process according to claim 1, 2, 3, 4 or 5, in which said second catalytically active material (4) comprises a platinum group metal, preferably palladium, in combination with one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, and niobium, on a support comprising silicon carbide or one or more oxides of metals taken from the group of aluminum, silicon and titanium.

7. A process according to claim 1, 2, 3, 4, 5 or 6, in which one or both of said first (2) and second (4) catalyt^¬ ically active material is provided as a monolith structure.

8. A process according to claim 1, 2, 3, 4, 5, 6 or 7, in which the total concentration of ¾S is less than 2.5 vol%.

9. A process according to claim 1, 2, 3, 4, 5, 6, 7 or 8, in which the following concentration condition is met c_Cs2 + 0.5 Ccos + 0.25 Ceo < 0.7 vol%

10. A process for production of sulfuric acid comprising the steps of one of claim 1, 2, 3, 4, 5, 6, 7, 8 or 9 and the further steps of

(c) directing said incinerated gas to contact a material (19,21) catalytically active in oxidation of sulfur dioxide to sulfur trioxide providing an oxidized gas (23) and

(d) at least partially hydrating sulfur trioxide forming sulfuric acid,

(e) withdrawing from said oxidized gas sulfuric acid (25) .

11. A process according to claim 10 further comprising the step of condensing sulfuric acid in a cooled condenser (26) .

12. A process according to claim 10 further comprising the step of absorbing sulfur trioxide in sulfuric acid.

13. A process plant configured for directing a gas to a catalyst system (2,4), characterized in said catalyst sys- tern (2,4) comprising a first material (2) catalytically active in oxidation of hydrogen sulfide but substantially catalytically inactive in oxidation of carbon disulfide, carbonyl sulfide and carbon monoxide and a second material (4) catalytically active in oxidation of carbon disulfide, carbonyl sulfide and carbon monoxide at a second higher temperature .

14. A process plant according to claim 13, in which one or both of said first or second catalytically active materials (2) comprise one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, and niobium, on a support comprising silicon carbide or one or more oxides of metals taken from the group of aluminum, silicon and titanium.

15. A process plant according to claim 13 or 14, in which said second catalytically active material (4) comprises a platinum group metal, preferably palladium in combination with one or more oxides of a metal taken from the group consisting of vanadium, chromium, tungsten, molybdenum, cerium, and niobium, on a support comprising silicon carbide or one or more oxides of metals taken from the group of aluminum, silicon and titanium.